The present invention relates generally to the field of radio communications. More specifically, the present invention relates to a transmitter architecture utilizing a voltage controlled attenuator to control its transmission signal level.
A new generation of radiotelephones is expected to be more capable and more efficient than a previous generation of radiotelephones. Such new radiotelephone may be expected to allow a user to operate in multiple systems that were traditionally incompatible such as time-division-multiple-access (TDMA), global system for mobile (GSM), and code-division-multiple-access (CDMA), while providing a longer talk time without increasing in size. In order to support multiple systems, the radiotelephone is likely to be required to support multiple radio frequency (RF) bands such as 800 MHz, 900 MHz, 1800 MHz, and 1900 MHz.
A voltage controlled attenuator (VCA) is an essential part of today""s transmitter architecture providing a strict control of a transmission signal level required by the systems. A common differential VCA 100 consists of a differential amplifier 102 and a steering block 104 as illustrated in FIG. 1. The signal level at the output 106 and 108 of the VCA is varied by applying a differential control voltage (Vc) to the steering lines 110 and 112 which are connected to the bases of transistor pair 114 and 116, and to the bases of transistor pair 118 and 120, respectively. The control voltage steers the current flow between 114 and 118, and between 116 and 120. For a maximum output power, a maximum differential control voltage is applied to the steering lines 110 and 112 with the line 110 having a lower voltage and the line 112 having a higher voltage relative to each other respect to a circuit ground. All of the current available from the power supply line 130 flows through loads 122 and 124, and a differential RF input signal present on lines 126 and 128 is amplified. For a minimum power, a maximum differential control voltage is applied to the steering lines 110 and 112 with the line 110 having a higher voltage and the line 112 having a lower voltage relative to each other respect to a circuit ground. The current available from the power supply line 130 is allowed to flows through transistor pair 114 and 116 thereby reducing the current flow through loads 122 and 124 and attenuating the differential RF input signal.
In order to support multiple bands, today""s common transmitter architecture employed in a radiotelephone uses a VCA followed by an external switch to select one of the multiple bands, or uses multiple VCAs dedicating one VCA for each band as illustrated. However, each of these methods has disadvantages.
The external switch method requires an additional component, namely a separate switch device. Because of system requirements for high linearity for high power signal, the switch device must achieve high linearity requiring large current drain, and the signal going through the switch device experiences some loss which degrades the signal quality. The through-loss of the switch device must be compensated with a higher gain, which requires more current drain, somewhere in the transmitter lineup in order to achieve a desired power output. The external switch device also requires an additional space on the radiotelephone printed circuit board making the radiotelephone larger.
The multiple VCA method does not degrade the signal quality. However, integrating multiple VCA circuits into a single device increases the die size of the device and increases parts required to support such circuits.
Accordingly, there is a need to control an output power level of a radio frequency (RF) signal and to direct the RF signal to an appropriate path for a multi-mode and multi-band radiotelephone without using an external switch and without using multiple common VCAs.